NMR

Basic Principle of NMR & uses

1. Nuclear Magnetic Resonance (NMR) Spectroscopy BY : Mr. VIKAS CHANDRA SHARMA Asst. Prof., Department of Chemistry Shri Shankaracharya Mahavidyalaya Junwani - Bhilai

2. NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY : Commonly referred to as NMR, is a technique which exploits the magnetic properties of certain nuclei to study physical, chemical, and biological properties of matter Compared to mass spectrometry, larger amounts of sample are needed, but non-destructive DEFINITION

3. Fermions : Odd mass nuclei with an odd number of nucleons have fractional spins. I = 1/2 ( 1 H, 13 C, 19 F, 31 P ), I = 3/2 ( 11 B, 33 S ) & I = 5/2 ( 17 O ). Bosons : Even mass nuclei with odd numbers of protons and neutrons have integral spins. I = 1 ( 2 H, 14 N ) Even mass nuclei composed of even numbers of protons and neutrons have zero spin I = 0 ( 12 C, and 16 O, 32 S) SPIN OF NUCLEI

4. A spinning charge generates a magnetic field, the resulting spin-magnet has a magnetic moment ( μ ) proportional to the spin I magnetic moment m = g p where g is the gyromagnetic ratio , and it is a constant for a given nucleus When I=0, m =0 “ Right Hand Rule” determines the direction of the magnetic field around a current-carrying wire and vice-versa ** There is no spin for nuclei with I=0 ANGULAR MOMENTUM

5. ENERGY DIFFERENTIATION In the presence of an external magnetic field ( B ), two spin states exist, +1/2 and -1/2 (For I=1/2). The magnetic moment of the lower energy +1/2 state is aligned with the external field, and that of the higher energy -1/2 spin state is opposed to the external field. Aligned against the applied field Aligned with the applied field

6. When the energy of the photon matches the energy difference between the two spin states , an absorption of energy occurs. We call that phenomenon Resonance Difference in energy between the two states is given by: D E = g h B o / 2 p where: B o – external magnetic field h – Planck’s constant g – gyromagnetic ratio D E = h u = g hB o / 2 p So, u = g B o / 2 p ENERGY DIFFERENTIATION

7. LARMOR PRECESSION Spinning particle precesses about the external field axis with an angular frequency known as the Larmor frequency w L = g B o When radio frequency energy matching the Larmor frequency is introduced at a right angle to the external field, it would cause a transition between the two energy levels of the spin. In other world, the precessing nucleus will absorb energy and the magnetic moment will flip to its I = _ 1/2 state

8. g - VALUES FOR SOME NUCLEI Isotope  Net Spin  g / MHz T -1 Abundance / % 1 H 1/2  42.58 99.98 2 H 1  6.54 0.015 3 H 1/2 45.41 0.0 31 P 1/2  17.25 100.0 23 Na 3/2  11.27 100.0 14 N 1  3.08 99.63 15 N 1/2 4.31 0.37 13 C 1/2  10.71 1.108 19 F  1/2  40.08 100.0

9. FOURIER TRANSFORMATION AND THE NMR SPECTRUM Fourier transform RF Pulse The Fourier transform (FT) is a computational method for analyzing the frequencies present in an oscillating signal The NMR spectrum

10. 1 H NMR spectra 13 C NMR spectra d ppm d ppm High field Down field 1 H NMR and 13 C NMR SPECTRUM

11. CHEMICAL SHIFT- d When an atom is placed in a magnetic field, its electrons circulate about the direction of the applied magnetic field. This circulation causes a small magnetic field at the nucleus which opposes the externally applied field The magnetic field at the nucleus (the effective field) is therefore generally less than the applied field by a fraction : B = B0 (1-s), So u = g B (1- s ) / 2 p

12. STANDARD FOR CHEMICAL SHIFT In NMR spectroscopy, the standard is often tetramethylsilane, Si(CH 3 ) 4 , abbreviated TMS. Tetramethyl silane (TMS) is used as reference because it is soluble in most organic solvents, is inert, volatile, and has 12 equivalent 1H and 4 equivalent 13C. TMS signal is set to 0

13. SHIELDING AND DESHIELDING A nucleus is said to be shielded when electrons around the nucleus circulates in a magnetic field and create a secondary induced magnetic field which opposes the applied field . Trends in chemical shift are explained based on the degree of shielding or deshielding , e.g. of deshielding effect

14. CHEMICAL SHIFT- d Chemical shift depends on : Electronegativity of nearby atoms Hybridization of adjacent atoms diamagnetic effects paramagnetic effects solvent effect

15. Spin-spin coupling: The coupling of the intrinsic angular momentum of different particles. Such coupling between pairs of nuclear spins is an important feature of nuclear magnetic resonance (NMR) spectroscopy as it can provide detailed information about the structure and conformation of molecules. Spin-spin coupling between nuclear spin and electronic spin is responsible for hyperfine structure in atomic spectra. SPIN-SPIN COUPLING

16. J -Coupling J -coupling: also called indirect spin-spin coupling , is the coupling between two nuclear spins due to the influence of bonding electrons on the magnetic field running between the two nuclei. J-coupling provides information about dihedral angles, which can be estimated using the Karplus equation. It is an important observable effect in 1D NMR spectroscopy. The coupling constant, J (usually in frequency units, Hz) is a measure of the interaction between a pair of nuclei

17. 1 H-NMR 1 H experiencing the same chemical environment or chemical shift are called equivalent hydrogens. 1 H experiencing different environment or having different chemical shifts are nonequivalent hydrogens.

18. CHEMICAL SHIFT - 1 H-NMR

19. 1 H CHEMICAL SHIFTS

20. Chemical shift : (1) electronegativity of nearby atoms, (2) hybridization of adjacent atoms, and (3) diamagnetic effects Electronegativity FACTORS TO AFFECT 1 H CHEMICAL SHIFT

21. Hybridization of adjacent atoms

22. Carbon-Carbon Triple Bond Effect A carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal to lower frequency (to the right) to a smaller value

23. CARBON-CARBON DOUBLE BOND EFFECT Magnetic induction in the p bond of a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal higher frequency

24. AROMATIC EFFECT The magnetic field induced by circulation of p electrons in an aromatic ring deshields the hydrogens on the ring and shifts their signal to higher frequency

25. SIGNAL SPLITTING FOR 1 H Peak: The units into which an NMR signal is split; doublet, triplet, quartet, multiplet, etc. Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens. ( n + 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1 H-NMR signal is split into ( n + 1) peaks.

26. The relative peak intensities for multiplet peaks arising from J- coupling of a 1 H to N equivalent 1 H can be determined using Pascal’s triangle: PASCAL’S TRIANGLE

27. COUPLING CONSTANT Coupling constant ( J ): The separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet .

28. 13 C-NMR SPECTROSCOPY Organic compounds contain carbon. Unfortunately, the C-12 nucleus does not have a nuclear spin, but the C-13 nucleus does due to the presence of an unpaired neucarbon-1tron. C-13 nuclei make up approximately 1% of the carbon nuclei on earth. Therefore, 13 C NMR will be much less sensitive than 1 HNMR NMR

29. 13 C-NMR SPECTROSCOPY The presence of spin-spin coupling between a 1 3 C nucleus and the nuclei of 1 H atoms bonded to the 13 C , splits the carbon-13 peaks and causes an even poorer signal-to-noise ratio Each nonequivalent 13 C gives a different signal A 13 C signal is split by the 1 H bonded to it according to the ( n + 1) rule. Coupling constants of 100-250 Hz are common, which means that there is often significant overlap between signals, and splitting patterns can be very difficult to determine . The most common mode of operation of a 13 C-NMR spectrometer is a proton-decoupled mode.

30. proton-decoupled mode , a sample is irradiated with two different radiofrequencies. One to excite all 13 C nuclei, a second to cause all protons in the molecule to undergo rapid transitions between their nuclear spin states. On the time scale of a 13 C-NMR spectrum, each proton is in an average or effectively constant nuclear spin state, with the result that 1 H- 13 C spin-spin interactions are not observed and they are decoupled. DECOUPLING

31. CHEMICAL SHIFT - 13 C-NMR Trends RCH 3 < R 2 CH 2 < R 3 CH Electronegative atoms cause downfield shift Pi bonds cause downfield shift C=O 160-210 ppm

32. 13 C-NMR: INTEGRATION 1 H-NMR : Integration reveals relative number of hydrogens per signal 13 C-NMR : Integration reveals relative number of carbons per signal Rarely useful due to slow relaxation time for 13 C time for nucleus to relax from excited spin state to ground state

33. INTERPRETING NMR SPECTRA Alkanes 1 H-NMR signals appear in the range of 0.8-1.7. 13 C-NMR signals appear in the considerably wider range of 10-60 . Alkenes 1 H-NMR signals appear in the range 4.6-5.7. 1 H-NMR coupling constants are generally larger for trans -vinylic hydrogens ( J= 11-18 Hz ) compared with cis -vinylic hydrogens ( J= 5-10 Hz ). 13 C-NMR signals for sp 2 hybridized carbons appear in the range 100-160, which is to higher frequency from the signals of sp 3 hybridized carbons.

34. INTERPRETING NMR SPECTRA Alcohols 1 H-NMR O-H chemical shift often appears in the range 3.0-4.0 , but may be as low as 0.5. 1 H-NMR chemical shifts of hydrogens on the carbon bearing the -OH group are deshielded by the electron-withdrawing inductive effect of the oxygen and appear in the range 3.0-4.0. Ethers A distinctive feature in the 1 H-NMR spectra of ethers is the chemical shift, 3.3-4.0 , of hydrogens on the carbons bonded to the ether oxygen .

35. a a b b

36. Aldehydes and ketones 1 H-NMR: aldehyde hydrogens appear at 9.5-10.1 . 1 H-NMR: a-hydrogens of aldehydes and ketones appear at 2.2-2.6 . 13 C-NMR: carbonyl carbons appear at 180-215 . Amines 1 H-NMR: amine hydrogens appear at 0.5-5.0 depending on conditions. INTERPRETING NMR SPECTRA

37. 1 H NMR isobutyraldehyde 1 H NMR Methyl ethyl ketone a b c b c a a a b c c b

38. INTERPRETING NMR SPECTRA Carboxylic acids 1 H-NMR: carboxyl hydrogens appear at 10-13 ppm , higher than most other types of hydrogens. 13 C-NMR: carboxyl carbons in acids and esters appear at 160-180 ppm . c c b b a a

39. REFERENCES: Organic chemistry by Morrison and Boyd, Prentice-hall PUBLICATIONS Organic chemistry by F.A. Carey , McGraw Hill PUBLICATIONS